1. Field of the Invention
The present invention relates to the art of axle/suspension systems for vehicles. More particularly, the invention relates to the art of trailing or leading arm air-ride axle/suspension systems for heavy-duty vehicles, such as tractor-trailers or semi-trailers, which cushion the ride and stabilize the vehicle during operation. Still more particularly, the invention relates to the art of trailing or leading arm air-ride axle/suspension systems for heavy-duty vehicles of the type that may be susceptible to roll forces and dock walk.
2. Background Art
Heavy-duty vehicles that transport freight, for example, tractor-trailers or semi-trailers and straight trucks, typically include leading or trailing arm air-ride suspension assemblies that connect the axles of the vehicle to the frame of the vehicle. These air-ride suspension assemblies use air springs to cushion the ride of the vehicle. In some heavy-duty vehicles, the suspension assemblies are connected directly to the primary frame of the vehicle. In other heavy-duty vehicles, the primary frame of the vehicle supports a subframe, and the suspension assemblies connect directly to the subframe. For those heavy-duty vehicles that support a subframe, the subframe can be non-movable or movable, the latter being commonly referred to as a slider box, slider subframe, slider undercarriage, or secondary slider frame. For the purpose of convenience and clarity, reference herein will be made to a slider box, with the understanding that such reference is by way of example, and that the present invention applies to heavy-duty vehicle primary frames, movable subframes and non-movable subframes.
In the heavy-duty vehicle art, one or more axle/suspension systems usually are suspended from a single slider box. It is understood that a slider box outfitted with usually two axle/suspension systems typically is referred to as a slider tandem, and for purposes of convenience and clarity, will hereinafter be referred to as a slider tandem. Of course, a slider box may also be outfitted with a single axle/suspension system, or three or more axle/suspension systems. By way of example, reference herein shall be made to a slider tandem having a pair of axle/suspension systems mounted thereon, with the understanding that such reference also applies to a slider outfitted with one, three or more axle/suspension systems. The slider tandem in turn is mounted on the underside of the trailer primary frame, and is movable longitudinally therealong to provide a means for variable load distribution and vehicular maneuverability.
More specifically, the amount of cargo that a trailer may carry is governed by local, state and/or national road and bridge laws, and is dependent on proper load distribution. The basic principle behind most road and bridge laws is to limit the maximum load that a vehicle may carry, as well as limit the maximum load that can be supported by individual axles. A trailer having a slider tandem gains an advantage with respect to laws governing maximum axle loads. More particularly, proper placement of the slider tandem varies individual axle loads or redistributes the trailer load so that it is within legal limits.
A slider box or other subframe typically includes a pair of longitudinally-extending, parallel, transversely-spaced elongated main members. A plurality of longitudinally-spaced parallel cross members extend transversely between and are attached to the main members. Pairs of transversely-spaced hangers are mounted on and depend from the main members and selected ones of the cross members. An axle/suspension system typically includes a pair of transversely-spaced trailing arm beams, each of which is pivotally connected at its front end to a respective one of the hangers. Each trailing arm beam also is welded or otherwise rigidly attached at its rear end to a transversely-extending axle of the axle/suspension system. The wheels of the vehicle are rotationally mounted, as known in the art, to opposing ends of the axle. The axle/suspension system further conventionally includes a pair of air springs, which each extend between and are mounted on the rear end of a respective one of the beams and a respective one of the main members, and a pair of shock absorbers, which each extend between and are mounted on a respective one of the beams and a respective one of the main members. It should be noted that, while the hangers are sometimes considered to be part of the vehicle frame once they are connected to the frame members, they are typically engineered as part of the axle/suspension system.
The axle/suspension system of the heavy-duty vehicle also acts to cushion the ride and stabilize the vehicle. More particularly, as the vehicle is traveling over-the-road, its wheels encounter road conditions that impart various forces, loads and/or stresses, collectively referred to herein as forces, to the respective axle on which the wheels are mounted, and in turn, to the suspension assemblies that are connected to and support the axle. In order to minimize the detrimental effect of these forces on the vehicle as it is operating, the axle/suspension system is designed to absorb at least some of them.
These forces include vertical forces caused by vertical movement of the wheels as they encounter certain road conditions, fore-aft forces caused by acceleration and deceleration of the vehicle, and side-load and roll forces associated with transverse vehicle movement, such as turning of the vehicle and lane-change maneuvers. In order to address such disparate forces, axle/suspension systems have differing structural requirements. More particularly, it is desirable for an axle/suspension to be fairly stiff or rigid to minimize the amount of sway experienced by the vehicle and thus provide what is known in the art as roll stability. However, it is also desirable for an axle/suspension system to be relatively flexible to assist in cushioning the vehicle from vertical impacts, and to provide compliance so that the components of the axle/suspension system resist failure. In order to resolve these differing structural requirements, prior art axle/suspension systems include undesirable excessive weight and cost, as will be described in detail below.
In addition, it is desirable for trailing arm axle/suspension systems to reduce or prevent an event known as “dock walk,” reduction of which typically has been achieved through the use of pivoted links and/or other components. More particularly, many heavy-duty vehicles transport dry freight, that is, cargo that is loaded into a van or trailer of a typical heavy-duty vehicle. To receive cargo, or to have it removed, the vehicle often parks at a loading dock, with a rear end of the trailer in close proximity to the dock. Due to the weight of the cargo, a fork lift or other transfer vehicle is used to load the cargo into or unload the cargo from the trailer, and travels from the loading dock into the trailer. At this stage of the loading or unloading process, a disadvantage of many prior art axle/suspension systems occurs, which is an event known in the art as “dock walk.” The dock walk event will be described in greater detail below, but may be summarized as a generally arcuate motion of the rigid trailing arm beam and axle of the axle/suspension system in response to the sudden weight increase of the fork lift driving into the trailer, which causes the vehicle tires to rotate in a forward direction and undesirably move the trailer away from the loading dock.
In an attempt to reduce or prevent dock walk in prior art rigid-beam leading or trailing arm air-ride axle/suspension systems, additional components have been used to reduce the arcuate motion of the trailing arm beam when a forklift or other device is introduced into the trailer during a loading or unloading situation. For example, some systems employ a mechanical stop or similar structural component, as will be described in greater detail below, which reduces arcuate motion of the beam and the axle, and in turn reduces the rotation of the tires, which minimizes dock walk. Other systems employ manual exhaust valves, which will also be described in greater detail below, which enable bumpers within the air springs to act as a positive mechanical support and minimize arcuate motion of the beam and the axle, in turn minimizing forward rotation of the tires and dock walk.
However, components such as a structural stop and/or exhaust valves, as well as associated components for the operation and control of the stop and/or valves, involve an undesirable increase in weight of the axle/suspension system, as well as an undesirable increase in cost for the system. Moreover, such additional components add to the complexity of the axle/suspension system, undesirably increasing maintenance costs for the system.
Other types of prior art air-ride axle/suspension systems that are known in the art also reduce or prevent dock walk, such as parallelogram linkages in which each beam is made up of links that are pivotally attached to the frame hanger and the axle, as will be described in greater detail below. However, such prior art parallelogram linkage axle/suspension systems possess a distinct disadvantage, which is a lack of stiffness or rigidity that creates an inherent lack of roll stability. In order to provide stability, an auxiliary roll bar assembly must be incorporated into the parallelogram linkage system, which involves the addition of multiple components and thereby undesirably increases the weight, complexity, cost and maintenance of the system.
Other types of prior art axle/suspension systems, which are known in the art as mechanical spring suspension systems, typically are not subject to dock walk. Spring suspension systems, however, are not air-ride systems, and instead include a pairs of transversely-spaced leaf springs. These leaf springs are engineered to carry the vertical load of the vehicle, and therefore typically are stiff enough to control roll forces. The stiffness of the leaf springs of the spring suspension creates a significant disadvantage for the suspension, since the leaf springs must be engineered to be stiff enough to provide vertical force resistance and thus roll stability for a fully-loaded trailer, which sacrifices flexibility in situations where the trailer is only lightly loaded, thereby creating an extremely harsh ride when the trailer is lightly loaded. Thus, the ride that is enabled by a spring suspension is considerably less than optimum.
Therefore, in the prior art, the competing demands of stiffness or rigidity for roll stability, flexibility for compliance and dampening of vertical forces, and/or movable attachment of beams or links to the frame hangers and/or the axle for reduction of dock walk, have led to axle/suspension systems that provide less-than-optimum reaction of forces, and which are undesirably heavy and expensive. As a result, a need has existed in the art to develop a leading or trailing arm air-ride axle/suspension system that overcomes the disadvantages of prior art systems and provides a relatively lightweight, simple and economical system that can provide vehicle roll stability while potentially reducing or eliminating dock walk.